The present invention relates to an organic electroluminescence element in which at least a hole injection layer and a luminous layer are interposed between a pair of electrodes.
Since an electroluminescence element (hereinafter referred to as an xe2x80x9cEL elementxe2x80x9d) using electroluminescence causes self-luminescence, the visibility is high and the element is a completely solid state element so that it has an excellent impact strength. Accordingly, its use as a luminescence element in various display devices has attracted much interest.
This EL element includes an inorganic EL element obtained by using an inorganic compound as a luminous material and an organic EL element obtained by using an organic compound as a luminous material. Especially in the organic EL element of these, the driving voltage is quite low as compared with the inorganic EL element, and the element can further easily be downsized. Accordingly, studies and developments to put the same to practical use have been increasingly conducted.
In order to put the organic EL element to practical use, the increase in efficiency of the element performance and the improvement in the driving life are indispensable. For solving these problems, the improvements in the luminous material and the construction of the element have been carried out.
As the organic EL element, known is an element based on a laminated-type element construction of anode/organic luminous layer/cathode and provided on this with a hole injection transfer layer or an electron injection transfer layer as required, for example, an element having the construction of anode/hole injection transfer layer/organic luminous layer/cathode or the construction of anode/hole injection transfer layer/organic luminous layer/electron injection transfer layer/cathode, or the like is known.
The hole injection transfer layer here has a function of injecting holes from an anode at good efficiency and transferring the holes to the luminous layer, and it is, in many cases, constructed of a hole injection layer and a hole transfer layer.
Further, the electron injection transfer layer has a function of injecting electrons from a cathode at good efficiency and transferring the electrons to the luminous layer.
The luminous layer has a function of conducting luminescence by recombination of holes and electrons injected.
As the anode of the organic EL element, a transparent electrode formed of an ITO (Indium Tin Oxide) film is generally used. In this case, in order to inject holes from ITO at good efficiency by reducing an energy barrier in the hole injection, an amine-type material of which the ionization potential is close to that of ITO to give a great degree of hole transfer is often used in the hole injection layer.
By the way, since the organic EL element has quite a low thickness of approximately 100 nm, the surface form of ITO greatly influences the performance of the element. Specifically, when a protrusion or the like is present on the surface of ITO, the crystallization of the organic thin film proceeds with this protrusion as a base point, and it causes increase in a leak current or formation of a non-luminous point called a dark spot. For this reason, a high amorphousness and good film properties are required for the hole injection layer formed on ITO.
Further, when the organic EL element is driven through constant current driving, the driving voltage is increased with time to decrease a luminance. Such a deterioration phenomenon is considered to occur because a chemical reaction such as an oxidation reaction or the like is caused in an interface between ITO and the hole injection layer which is directly contacted with this ITO to proceed with the deterioration through the driving.
In order to solve such a problem, an organic EL element in which a hole injection transfer zone is constructed of a layer containing a hole-injecting porphyrin compound and a hole-transferring aromatic tertiary amine is disclosed (U.S. Pat. No. 2,597,377).
Further, it has been known that a CuPc (copper phthalocyanine) thin film is formed on the surface of ITO whereby a driving stability can be increased to reduce the increase in the driving voltage (S. A. Van Slyke et al., Appl. Phys. Lett., 69, 2160 (1996)).
Japanese Patent Laid-Open No. 314594/1994 discloses a structure in which a CuPc film is formed on ITO and a hole injection layer formed of a TPD-based oligomer, a triarylamine derivative is laminated on this CuPc film.
Although the driving stability is improved by these methods, there was a problem that a driving life (durability) cannot be improved satisfactorily.
Further, when pulse driving such as simple matrix driving is employed as a driving system in applying an organic EL element to a dot matrix display or the like, a current density has to be increased for conducting luminescence instantaneously at high luminance. Accordingly, there arises a need to pass a great current by periodically applying a high voltage. As a result, an element comes to be driven under severer conditions than in case of ordinary DC (direct current) driving. Therefore, the pulse driving involved a problem that a chemical reaction in an interface between ITO and a hole injection layer tends to proceed to give a short life.
Further, in order to put a luminescence element to practical use, a stability at a high temperature is required. However, when an ordinary organic EL element is stored at a high temperature, there are problems that the efficiency tends to decrease, further luminescence becomes uniform and the like. Thus, it was difficult to put the same to practical use.
It is an object of the present invention to provide an organic EL element in which a durability by which to endure the pulse driving can be secured and the heat resistance is excellent.
The present inventors have assiduously conducted investigations, and have consequently obtained findings that an intermediate layer is interposed between a hole injection layer and an anode and a material to meet predetermined conditions is used in the hole injection layer, making it possible to realize the prolongation of life and the improvement in the heat resistance. The present invention has been completed on the basis of these findings.
That is, the gist of the present invention is as follows.
1. An organic electroluminescence element comprising an anode and a cathode which are opposite to each other, and a hole injection layer and a luminous layer which are interposed between these anode and cathode, characterized in that the hole injection layer contains an oligomer having a phenylenediamine structure and having a glass transition temperature of 110xc2x0 C. or more, and an intermediate layer for inhibiting a reaction in an interface between the hole injection layer and the anode is formed between the hole injection layer and the anode.
2. The organic electroluminescence element as recited in the above 1, wherein an ionization potential of the intermediate layer is larger than a work function of the anode and smaller than an ionization potential of the oligomer of the hole injection layer.
3. The organic electroluminescence element as recited in the above 1, wherein the intermediate layer is formed of an inorganic semiconductor.
4. The organic electroluminescence element as recited in the above 2, wherein the intermediate layer is formed of an inorganic semiconductor.
5. The organic electroluminescence element as recited in the above 1 or 2, wherein the intermediate layer is formed of an inorganic insulator.
6. The organic electroluminescence element as recited in the above 1 or 2, wherein the intermediate layer is formed of a phthalocyanine-based compound.
7. The organic electroluminescence element as recited in the above 1 or 2, wherein the intermediate layer is formed of a carbon film.
The present invention is an organic electroluminescence element comprising an anode and a cathode which are opposite to each other, and a hole injection layer and a luminous layer which are interposed between these anode and cathode, characterized in that the hole injection layer contains an oligomer having a phenylenediamine structure and having a glass transition temperature of 110xc2x0 C. or more, and an intermediate layer for inhibiting a reaction in an interface between the hole injection layer and the anode is formed between the hole injection layer and the anode.
The hole injection layer here is a layer which is formed between the anode and the luminous layer for improving the injection property of the hole.
Further, the phenylenediamine structure here is a structure that two amines are arranged through a phenyl group.
Examples of the material having this structure include, for example, a compound represented by general formula (I) 
(wherein n is an integer of 1 to 3, Ar1 to Ar7 each represent a carbocyclic group having 6 to 30 carbon atoms, and either Ar2 or Ar5 is a phenylene group), a compound represented by general formula (II) 
(wherein m is an integer of 1to 3, Ar1 to Ar9each represent a carbocyclic group having 6 to 30 carbon atoms, and at least one of Ar2, Ar4 and Ar5 is a phenylene group), and the like.
In the present invention, among the oligomers having such a phenylenediamine structure, one having a glass transition temperature of 110xc2x0 C. or more is used as the material for the hole injection layer, so that the heat resistance of the element can markedly be improved and the excellent luminous efficiency is obtained.
And, the intermediate layer is formed between the hole injection layer containing such an oligomer and the anode, whereby the hole injection layer and the anode can be spaced apart, making it possible to eliminate the chemical reaction in the interface between the hole injection layer and the anode. Consequently, the excellent durability by which to endure the severe driving conditions in the pulse driving or the like can be secured, and the prolongation of life of the element can be achieved.
The hole injection layer may be interposed between the anode and the luminous layer along with the hole transfer layer, and may have a function as a hole injection layer that improves both the injection property and the transferability of the hole.
The above-described oligomer may be a linear oligomer or a branched oligomer.
In this case, it is desirable that the ionization potential of the intermediate layer is larger than a work function of the anode and smaller than the ionization potential of the oligomer of the hole injection layer.
When the ionization potential of the intermediate layer is thus defined, the hole injection barrier can surely be decreased, making it possible to decrease the driving voltage and to improve the durability.
The material constituting the intermediate layer may be either an organic material or an inorganic material. For example, the intermediate layer can be formed of an inorganic semiconductor or an inorganic insulator. Examples of the inorganic semiconductor include, for example, GaAlN, GaInN, GaN, Sixxe2x88x921xe2x80x94Cx (0 less than xc3x97 less than 1), Si, CuI, ZnTe, ZnS, CdS, CdTe, CdSexS1xe2x88x92x(0 less than xc3x97 less than 1), Te and Se. Examples of the inorganic insulator include SiOx (0 less than xc3x97 less than 2), LiF, Li2O, Al2O3, TiO2, BaF2, CaF2, MgF2 and the like.
In case the organic material is used as the material of the intermediate layer, the intermediate layer can be formed of a phthalocyanine-type compound, a quinacridone-type compound or the like.
Further, the intermediate layer can also be formed of a carbon film, and it can specifically be formed of p-type diamond, a diamond-like carbon film (SP3 ingredient-containing carbon film) or the like.